(1) Field of the Invention
The present invention relates generally to fluid clarification systems and, more particularly, to a system and method of solids from a slurry, particularly drilling mud which has already been clarified.
(2) Description of Related Art
The present invention provides a centrifugal separation system which may be used on a drilling rig. When an oil well is drilled, it is necessary to drill the well with drilling fluid, commonly referred to in the art as drilling mud. The drilling mud is provided to lubricate and cool the drill bit and to carry away cuttings as the mud flows upwardly in the annular flow space around the drill string. The drilling mud is pumped down the drill string to pick up the cuttings and other debris. Commonly, the drilling mud is water but it is sometimes made with an oil, an oil-based carrier, or a diesel-based fluid.
While circulating through the well, drilling mud picks up particles of the earth formations cut by the drill bit. It is relatively easy to clean the drilling mud if the cuttings are primarily heavy rock. Also, large particle cuttings are easily removed from the mud by passing the drilling mud through a set of screens. In general, as mud is returned to the surface, it typically flows into a mud pit and then is pumped out of the mud pit by a mud pump. While flowing from the well to the mud pit and then back to the mud pump, the mud typically is treated by a number of devices to restore the mud to its original condition, such devices including shale shakers, desanders, degassers, and other cleaning devices. The present invention is particularly adapted to receive wet drill cuttings from a shale shaker, but may also be positioned downstream from a centrifugal separator or separator system.
At times, the mud will simply be permitted to sit in an open pit. This enables the heavy particles in the mud to settle to the bottom. Gas bubbles also are removed so that entrained gas bubbles do not create a risk of explosion by accumulating odorless natural gas around the mud pits. Drilling mud with such entrained gas is also too light for almost all applications.
As previously described, drilling mud returning from down hole comprises a fluid such as water or a synthetic oil, high gravity materials added to the drilling mud, and low gravity solids (i.e., cuttings) from the drilling operation. The drilling mud is typically run through one or more centrifuges to remove as much of the cuttings as possible so that the drilling mud can be recirculated. After removing as much of the liquid as possible so that it is reclaimed, there remains a slurry of discharge solids with some entrained liquids remaining. Even the most efficient known centrifugal systems leave liquids in the slurry, typically 10% or more by weight. Recent changes in regulations from the Environmental Protection Agency specify that the solids discharge from offshore drilling platforms can contain no more than 4.3% by weight of oil based drilling mud. As previously mentioned, the cuttings dryer of this invention may receive such a slurry from a centrifuge and dry the cuttings in conformity with this or even more stringent requirements.
One known system for drying the slurry, whether from a centrifuge system or from a shaker on an offshore rig, includes a spinning frustoconical basket into which the slurry is directed. The slurry enters the basket at the bottom, narrow part of the cone, where centrifugal forces sling the slurry toward the wider part of the cone and away from the axis of rotation of the basket. Typically, the slurry remains in the basket for roughly three seconds, during which time some of the liquid from the slurry passes through the basket for recycling, while the now drier slurry is captured for disposal. Unfortunately, the length of time that the slurry remains in the basket is insufficient to adequately dry the slurry. If the basket is made longer, in order to retain the slurry in basket for a longer period, then the basket and accompanying drive system become so large that placement of the system on an offshore rig where space is at a premium becomes problematic. Also, with such a large system, the available spots on an offshore rig where the system can be positioned, and still be located close to where the slurry is being generated is also difficult. If the system cannot be placed close to the initial drying system which generates the slurry, then some sort of transport system must be developed at significant expense.
Another known centrifugal system is adapted for classifying cuttings. This system generally includes a cylindrical portion and a frustoconical portion, with the region of the bowl around the cylindrical portion defining a pool, and the region of the bowl around the frustoconical portion defining a beach. A screw conveyor picks up cuttings separated by centrifuge force into the bottom of the pool, and drive the cuttings up onto the beach for discharge. Such a system has not previously been adapted for drying cuttings from a slurry, not is it well adapted for this application.
Thus, there is a pressing need for a drying system which can effectively receive the slurry from a clarification system such as a shaker or a centrifuge and further remove liquid from the slurry. Such a system should be compact, so that it can be positioned within the limited space of an offshore platform, without the need for expensive and cumbersome transport systems. Also, the system should be capable of providing a solids discharge within EPA regulations.
The invention of the present invention is directed to this need in the art.
The present invention provides a method and apparatus for receiving the solids discharge of a centrifuge or other clarification system and further drying the solids discharge. The influent into the apparatus of the invention is in the form of a slurry, such as for example the solids discharge of centrifuge in a drilling mud clarification system. The apparatus comprises a horizontal screen rotated at a high rate of speed to remove liquid from the slurry by centrifugal force. The horizontal screen is retained by a squirrel case rotor so that high G-forces can be obtained. Within the horizontal screen is a screw conveyor while rotates at a speed slightly slower than the horizontal screen to define a controllable conveyor speed. In this way, the residence time of the solids in contact with the horizontal screen can be controlled to provide a drier solids discharge from the apparatus. The solids and liquids are provided with separate discharge ports.
The liquid discharge from the apparatus is recirculated to the drilling mud system for recirculation downhole. The solids discharge has less than 4.3% entrained liquid, within regulations for discharge to the environment.
It is thus an object of the invention to provide an improved, self-contained slurry processing system to further remove solids from drilling mud or other liquid influent having entrained solids. The system is compact so that it can more easily be installed in small spaces close to earlier stages of a purification system, thereby reducing the complexity and cost of transporting the slurry to the dryer. The system includes a control system to effectively control the residence time of the solids in contact with the horizontal screen to control the quantity of liquids which are removed from the slurry.
These and other features of the invention will be apparent to those skilled in the art from a review of the following detailed description along with the accompanying drawings.